Optical equalizer for signal-transmission system using multimode light guides

ABSTRACT

Signals, especially short pulses, are transmitted by way of a series of light guides with internally reflecting boundaries (i.e. fibers or foils) angularly adjoining one another, each junction being encased in an external body having the same index of refraction as the light guides. Within the body the light rays exiting from one guide are intercepted by one or two mirrors reflecting them to the other guide, each mirror having a cross-section in the form of a segment of an ellipse whose foci are the center points of the two guide ends in a common axial plane of the guides. Each segment terminates in two points represented by the intersection of a respective guide axis with the path of a limiting ray from the other guide.

FIELD OF THE INVENTION

Our present invention relates to an optical equalizer for a system inwhich signals, especially binary ones represented by short spikes(so-called Dirac pulses), are transmitted with the aid of multimodelight guides.

BACKGROUND OF THE INVENTION

A light guide as herein contemplated may be either a single opticalfiber, a group of optical fibers arrayed in a flat bundle or ribbon, ora light-conducting foil. In each instance the light guide has internallyreflecting boundaries with a critical angle of reflection determined bythe difference between the refractive indices of the guide substance andthe surrounding medium. As is well known, light rays striking the guideboundary at a glancing angle, not exceeding the critical value, aretotally reflected and thus do not leave the confines of the guide. Intraveling along their transmission path, they bounce back and forthbetween opposite guide surfaces and eventually leave the exit end of theguide at an inclination to its axis which depends upon the angle ofincidence.

Theoretically, at least, a ray may pass along the axis of a straightguide without internal reflection. Such a ray has the shortest transittime through the guide in comparison with rays undergoing reflection,the longest time being that of a ray exiting from the guide surface atthe critical or guidance angle. These relative delays of light raysoriginating at a common modulated source result at the receiving end ina broadening of the pulses and thus in a distortion of the signal. Thatdistortion, of course, increases with the length of the transmissionpath.

A variety of equalizers have already been suggested for dealing withthis problem. One such equalizer, described by D. C. Gloge in an articleentitled "Fiber-Delay Equalization by Carrier Drift in the Detector",Optoelectronics, vol. 5, 1973, pages 345 - 350, operates electronicallyon the electric pulses derived from the luminous signal at the receivingend; the light rays emerging at different angles from the exit end of anoptical fiber are electronically detected in separate zones working intodelay lines which introduce compensatory differences in transit time.Such a system, requiring active electronic components, is relativelycomplex and limited to specific radiation receivers.

Other solutions, such as those suggested in U.S. Pat. Nos. 3,759,590 and3,832,030, provide optical equalizers with refractive cones or lensesserving for a compensatory refraction of light rays incident atdifferent angles; these refractive elements must be inserted atintermediate points of the signal path and their presence entails anunavoidable loss of luminous energy. Because of their rigid structure,they introduce an invariable corrective factor of 4 representing thereciprocal of the ratio between the widths of a corrected light pulse inthe output of the equalizer and an incident light pulse in its input.

OBJECTS OF THE INVENTION

The general object of our present invention is to provide an improvedoptical equalizer for the purpose described in which this correctivefactor can be significantly increased, e.g. up to a value of about 50.

Another object is to minimize the loss of luminous energy in such anequalizer.

A further object is to provide an optical equalizer of this characterwhich is of simple construction and can be adapted to variousrequirements.

SUMMARY OF THE INVENTION

In accordance with our present invention, a first and a second lightguide extending in cascade between a transmitter and a receiver ofluminous signals have proximal ends respectively emitting and collectingbundles of light rays whose axes angularly intersect in a common plane,each of these bundles being bounded by a pair of limiting rays onopposite sides of its axis with which these rays include theaforedescribed critical angle. The rays exiting from the first lightguide are intercepted by one or two mirrors reflecting them to thesecond light guide, the cross-section of each mirror in the common axialplane having the shape of a segment or an ellipse whose foci are thecenter points of the two proximal guide ends within that plane; eachreflecting segment extends between the intersection of the axis of thefirst light guide with the position of the limiting ray of the secondlight guide and the intersection of the axis of the second light guidewith the position of the limiting ray of the first light guide.

In this way, as more fully explained hereinafter, rays arriving axiallythrough the first guide pass at the critical angle into the second guidewhereas those issuing at the critical angle from the first guide areaxially introduced into the second guide; if the guides are of equallength, the two rays will undergo approximately the same number ofinternal reflections along the combined path. Intermediate rays, exitingand re-entering at less than the critical angle, experience a combinednumber of reflections in the two guides which establishes for them atransit time close to that of the axial and limiting rays referred to.

It should be understood that the light rays passing through a systemaccording to our invention need not necessarily lie in the visiblespectrum. Naturally, two or more equalizers according to our inventionmay be used between successive cascaded light guides representingrespective sections of a transmision path.

According to another feature of our invention, the confronting ends ofthe cascaded light guides and the associated mirror or mirrors areembedded in a body of transparent material whose index of refractionsubstantially equals that of the light guides, thereby eliminating orminimizing further refractions at the fiber ends. Naturally, the mirroror mirrors should have a different refractive index.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of our invention will now be described indetail with reference to the accompanying drawing in which:

FIG. 1 is a somewhat diagrammatic view of confronting extremities of apair of cascaded light guides and two associated mirrors according toour invention;

FIG. 2 is a set of graphs relating to the operation of the system ofFIG. 1;

FIG. 3 is a perspective view of the extremities of the two light guidesof FIG. 1, here shown as fibers, together with one of the associatedmirrors;

FIG. 4 is a perspective view generally similar to FIG. 3 but showing thelight guides in the form of foils with a modified mirror;

FIG. 5 is a view similar to FIG. 1, showing the elements of the systemencased in a transparent body; and

FIG. 6 illustrates a modification of the system of FIG. 5 with a changein the relative inclination of the two light guides and with omission ofone of the mirrors.

SPECIFIC DESCRIPTION

In FIG. 1 we have illustrated parts of two light guides f₁, f₂, guide f₁conducting luminous radiation from a nonillustrated source toward lightguide f₂ for transmission to a load also not shown. Guide f₁ has an exitend whose central point F₁ is considered the origin of three rays a₁,b₁, c₁ issuing from that guide, ray a₁ being axially oriented andincluding with the other two rays a critical angle θ determined by therefractive index of the guide; it is assumed that this index is the samefor both light guides f₁ and f₂. Analogously, guide f₂ has an entranceend whose central point F₂ collects a bundle of rays, among them anaxial ray a₂ and two marginal rays b₂, c₂ including the critical angle θwith ray a₂. The two guide axes, and therefore rays a₁ and a₂, includewith each other an angle Φ which is independent of θ and is here shownto be about π/2.

The paths of rays a₁ and c₂, lying in a common axial plane of theguides, intersect in a point A₁ ; another point A₂ represents theintersection of rays c₁ and a₂ also lying in that axial plane. Points A₁and A₂ lie at the ends of a mirror S which is curved within that planealong a segment of an ellipse having its focal points at F₁ and F₂.Thus, light exiting axially from guide f₁, following the path of ray a₁,is reflected by mirror S along the path of ray c₂ to enger the guide f₂at the critical angle θ. Conversely, light exiting from guide f₁ at thecritical angle, following the path of ray c₁, is reflected by mirror Salong the path of axial ray a₂ toward the guide f₂. If the two guidesare of the same length, the transit time of the ray path a₁ + c₂ willsubstantially equal the transit time of the ray path c₁ + a₂. Thetransit time for intermediate rays (not shown), striking the mirror Sbetween points A₁ and A₂, will also be approximately the same.

As further shown in FIG. 1, another mirror T has an end B₁ at theintersection of the paths of rays b₁ and a₂ and an end B₂ at theintersection of the paths of rays a₁ and b₂. In this instance, too,luminous energy following the ray paths b₁ + a₂ and a₁ + b₂ will havepractically the same transit time and will therefore experiencevirtually identical phase shifts; again, the intermediate rays will havetransit times and phase shifts of approximately the same magnitude.

As will be readily apparent from FIG. 1, the path length of the raysreflected by mirror S will differ from that of the rays reflected bymirror T. Thus, the use of both mirrors together does not fullyeliminate the phase difference among the various rays traveling from thesource to the load. To improve the aforementioned corrective factor incomparison with the known equalization systems referred to, only onemirror -- preferably the inner mirror S -- should be used. On the otherhand, the simultaneous utilization of both mirrors is indicated where aminimum loss of radiant energy is the paramount consideration. Byselectively blocking and unblocking the passage of light to or from oneof the two mirrors we can, accordingly, adapt the system to differentoperational requirements.

It will further be evident that intersections A₁ and A₂ will vanish whenthe angle Φ approaches the value π - θ, thereby eliminating thepossibility of utilizing the mirror S; conversely, when that angle Φ isreduced to approximately the value of θ, the intersections B₁ and B₂disappear so that it will not be possible to employ the mirror T. Midwaywithin that range, i.e. for Φ = π/2, the corrective factor has the samevalue E = 4 as in the conventional refractive equalizers; this appliesto the use of either mirror S, T. In FIG. 2 we have potted thereciprocal K = 1/E of this factor, i.e. the width ratio between acorrected and an uncorrected pulse, against the axial angle Φ. Curves αand β in FIG. 2 relate to the use of mirror S alone, curve αrepresenting a critical angle θ = 0.3 rad. whereas curve β is for θ =0.1 rad. Curves α' and β' represent the corresponding values of θ withthe sole use of mirror T. Since a reduction of θ increases the distancebetween points A₁, A₂, on the one hand, and points B₁, B₂, on the otherhand, the corrective factor E decreases if the two mirrors are employedconjointly. In the lower part of the range, the use of mirror S aloneresults in a sharp rise of factor E to a value of about 50 for Φ ≈ θ; inthe nonillustrated upper part of the range, curves α, β aresubstantially the mirror images of the illustrated portions of curvesα40 , β', and vice versa.

Since the compensation of phase differences by an equalizer according toour invention applies only to straight light guides f₁, f₂, thetransmission of signals between a point of origin and a point ofdestination can occur only over a zig-zag path with two or moreangularly adjoining guide segments. Thus, a reduction of axial angle θnecessitates either a lengthening of the segments or an increase in thenumber of such segments and therefore also in the number of equalizers.For this reason, there are practical limits for the minimum value of θ.

The guides f₁ and f₂ of FIG. 1 could be individual fibers, as moreclearly shown in FIG. 3, yet the principles just described also apply toflat guides f₁ ' and f₂ ' as shown in FIG. 4, i.e. ribbons or foils. Inthe first instance, the reflecting surfaces of mirrors S and T may bepart of ellipsoids of revolution as illustrated for mirror S in FIG. 3.In the second instance, the mirror surfaces may be segments ofelliptical cylinders with generatric parallel to the minor sides of theguide sections, as shown at S' in FIG. 4; central points F₁ and F₂ ofFIG. 1 then represent short transverse lines (designated F₁ ' and F₂ 'in F16-4) which direct the ends of guides f₁ ' amd f₂ '.

An ellipsoidal mirror as shown in FIG. 3 may be replaced by two (ormore) cylindrically curved mirrors, similar to mirror S of FIG. 4,inclined toward one another to intercept rays deviating from the axialplane of the guides.

In FIGS. 5 and 6 we have shown the light guides f₁ and f₂ of FIG. 1embedded, together with mirrors S and T, in a body P of transparentmaterial whose refractive index is the same as that of the guides. FIG.5 shows a relatively large angle Φ > π/2, with two mirrors S and T,whereas FIG. 6 illustrates a small angle Φ ≈ 20, with only one mirror S.Naturally, flat light guides f₁ ' and f₂ ' of FIG. 4 can be similarlyencased. If body P consists not of a solid but of a refractive fluid,its presence will not interfere with changes in the acial angle Φ and inthe position of the mirror or mirrors.

An equalizer consisting of one or two mirrors, pursuant to ourinvention, constitutes much less of an encumbrance for an opticalsignaling system of the type here described than does an array of lensesor other refractive components.

We claim:
 1. A system for the optical transmission of signals,comprising:a first and a second light guide with internally reflectingboundaries respectively connected to a transmitter and a receiver ofluminous signals, said light guides having proximal ends respectivelyemitting and collecting bundles of light rays with axes angularlyintersecting in a common plane, each bundle being bounded by a pair oflimiting rays on opposite sides of the respective axis; and a mirrorconfronting said proximal ends for reflecting light rays from said firstlight guide to said second light guide, said mirror having across-section in said common plane in the shape of a segment of anellipse whose foci are the center points of said proximal ends withinsaid common plane, said segment extending between the intersection ofthe axis of said first light guide with the position of a limiting rayof said second light guide and the intersection of the axis of saidsecond light guide with the position of a limiting ray of said firstlight guide.
 2. A system as defined in claim 1, further comprising abody of transparent material having substantially the same index ofrefraction as said light guides, said proximal ends and said mirrorbeing embedded in said body.
 3. A system as defined in claim 1,comprising another mirror confronting said proximal ends for reflectinglight rays from said first light guide to said second light guide, saidother mirror having a cross-section in said common plane in the shape ofa segment of an ellipse whose foci are said center points and whichextends between the intersection of the axis of said first light guidewith the position of the other limiting ray of said second light guideand the intersection of the axis of said second light guide with theposition of the other limiting ray of said first light guide.
 4. Asystem as defined in claim 1 wherein said light guides are twoindividual fibers, the reflecting surface of said mirror being a segmentof an ellipsoid of revolution.
 5. A system as defined in claim 1 whereinsaid light guides have a width transverse to said common plane less thantheir width in said common plane, the reflecting surface of said mirrorbeing a segment of an elliptical cylinder with generatricesperpendicular to said common plane.